The MAX17557 operates in discontinuous mode (DCM), which means it cannot sink a significant amount of current when VOUT is pulled up by an external source. The controller will see that the voltage at FB is greater than the internal reference voltage (i.e. VFB) and stop sending energy to the output. The MAX17557 will wait for FB to fall to a level where switching is needed, then commence switching to support VOUT. There is also an overvoltage (OV) monitor that will assert PGOOD when the OV threshold (i.e. OV_UV) has been exceeded.